<div id="app">
  <comp></comp>
</div>

<script src="http://unpkg.com/vue@3"></script>
<script>
  Vue.createApp({
    components: {
      comp: {
        template: `
          <div>root1</div>
          <div>root2</div>
        `
      }
    }
  }).mount('#app')
</script>